This invention relates to improvements in a method for the transfer of polymer slurries and, more particularly, to a method for the continuous transfer of a fluid slurry composed of solid polymer particles and a diluent wherein the polymer slurry kept under pressure is continuously passed at a controlled flow rate through two heating zones to vaporized the diluent therefrom and then is introduced into a separation zone to separate the solid polymer particles from the vaporized diluent.
Prior to this invention, it had been proposed (U.S. Pat. Nos. 3,285,899 and 3,428,619) that a polymer slurry should be transferred through a flash tube adapted to separate the solid polymer particles from the diluent.
More specifically, U.S. Pat. No. 3,285,899 discloses a method for the transfer of a polymer slurry composed of solid polyolefin and a diluent, wherein the pressurized polymer slurry is introduced into a sealed heating zone of lower pressure and gradually increasing cross-sectional areas to vaporize the diluent from the polymer slurry. The resulting dispersion of substantially dried polyolefin particles in the diluent vapor is so treated as to isolate the solid polyolefin. It is evident from the description of the example of U.S. Pat. No. 3,285,899 that the flow rate of the polyolefin slurry introduced into the heating zone is regulated by means of an intermittently-actuated valve installed immediately upstream of the heating zone. However, the polyolefin slurry is intermittently introduced into the heating zone because the aforesaid valve is controlled by the on-off action of a timer. As a result of such intermittent introduction, the pressure and flow rate of the stream leaving the heating zone downstream of the valve tend to pulsate considerably.
U.S. Pat. No. 3,428,619 discloses improvements in a similar method of transfer of a polymer slurry. Also in this method, the polymer slurry is introduced into a heating zone by way of an intermittently actuated valve, the supply of polymer slurry being regulated by varying the period of its opening and closing cycles. Accordingly, the pressure and flow rate of the stream leaving the heating zone downstream of the valve still tend to pulsate heavily with time.
When the transfer of polymer slurries is carried out on an industrial scale, the above-described heavy pulsations of the stream leaving the heating zone bring about the following disadvantages.
Firstly, since the diluent vapor separated from the solid polymer particles is cooled and then compressed for liquefaction, a surge tank of very large size is required in order to minimize the effect of the pulsations and to even the load on the compressor.
Secondly, the intermittently actuated valve for regulating the flow rate of the polymer slurry can easily be damaged because it opens and closes with high frequency and the difference between its upstream and downstream pressures is great. Moreover, while the valve is closed, the deposited solid polymer particles, swollen with the diluent, melt on the inner wall of the heating zone, which thus tends to clog. This tendency increases especially when copolymers of low softening point are handled.
Thirdly, since the polymer slurry is intermittently supplied to the heating zone, the zone is loaded with slurry only for a fraction of the time and, therefore, a large-size apparatus is required.